PROJECT SUMMARY Small cell lung cancer (SCLC) is remarkable for exceptionally high metastatic potential, initial robust response to DNA damaging agents, and near universal development of resistance. This combination of predilection for early metastasis acquired treatment resistance ? often times manifested as cross resistance to multiple agents ? highlights a critical need for novel systemic therapies operating through a novel mechanism in order to achieve improved patient outcomes. Delta-like ligand 3 (DLL3), has recently been identified as a therapeutic target in SCLC. The highly tumor-selective surface expression of this protein make it an excellent candidate target for an antibody drug conjugate (ADC). Rovalpituzumab tesserine (Rova-T) is one such ADC that is showing encouraging efficacy signals in the clinic. However, despite apparent clinical benefit, this agent has also been associated with some severe adverse events attributable to the presence of the anthracycline PBD warhead. DLL3 targeting approaches are in need of both a real-time, quantitative diagnostic biomarker and a therapeutic approach with reduced toxicity. We propose a theranostic approach comprising on 89Zr immunoPET and a 90Y/177Lu radioimmunotherapeutic. The first Aim improves upon already promising bioconjugation chemistry. We are already able to obtain high-contrast immunoPET images using non-specific amine labeling and site-specific maleimide bioconjugation. We will improve upon this approach by developing more stable thiol-clickable methylsuflone chelators for 89Zr and 90Y/177Lu to minimize kidney dose. The second Aim identifies preclinical dosing parameters and comprehensively optimizes efficacy and toxicity in a traditional cell line xenograft. Our preliminary imaging data is focused on H82, an SCLC cell lined derived from a chemoexperienced patient. This cell line is very resistant to etoposide in vitro and in vivo and will be used to identify a radiotherapy dose that demonstrates efficacy in vivo, while minimizing dose to the kidney. Different dose ranges and schedules will be explored. The final Aim explores radiotherapy in a variety of in vivo contexts including lesion sizes ranging from 0.1 to 10 mm in diameter and representing both chemonave and chemoresistant disease. Radioisotopes have different energy deposition depending on the volume of the tumor being targeted. We will evaluate 90Y and 177Lu radioisotopes in different in vivo models of small cell lung cancer for the ability to eradicate lesions of different sizes. A unique resource in the lab is our collection of 10 paired chemonave and chemoresistant patient-derived xenograft lines. We will place particular emphasis on establishing efficacy in the context of acquired chemoresistance. Data obtained in this study should provide preclinical evidence in support of clinical translation of a DLL3 targeting theranostic based on rovalpituzumab.